Acetic acid is a basic chemical having a wide range of applications such as raw materials for polyvinyl acetate, acetylcellulose and acetic acid esters, as well as solvents for terephthalic acid production plants. As a method for producing acetic acid, a so-called Monsanto's method (or a methanol carbonylation method) is known in which methanol is carbonylated by carbon monoxide in the presence of a rhodium catalyst or the like to produce acetic acid.
In the production of acetic acid by the carbonylation of methanol, a trace amount of acetaldehyde is by-produced. In addition, it is known that when low boiling components useful for the carbonylation reaction of methanol, such as methyl acetate and methyl iodide, are tried to be recovered, and returned to a carbonylation reaction step of methanol to be effectively reused, acetaldehyde is accumulated in the system. Acetaldehyde causes a condensation reaction under the carbonylation condition and is converted into unsaturated aldehydes having a high boiling point. These unsaturated aldehydes are precipitated in the lower portion of a distillation column or the like to cause the problem of blockage or the like, along with further progress of condensation. In addition, crotonaldehyde or the like has a boiling point close to that of acetic acid, and thus is difficult to be separated in a purification step of acetic acid and is incorporated into an acetic acid product, resulting in the deterioration in quality of acetic acid particularly as a reducing substance.
As the method for removing acetaldehyde accumulated in the system, PTL 1 discloses a method in which a mixed liquid of acetaldehyde, methyl acetate, acetic acid, methyl iodide and water is distilled using a distillation column having 40 steps or more in a reflux ratio of 10 or more to separate and remove acetaldehyde. In addition, PTL 2 discloses a method in which vapors of methyl iodide (boiling point: 42.4° C.), methyl acetate (boiling point: 56.9° C.) and acetaldehyde (boiling point: 20.2° C.) are brought into contact with water to thereby remove acetaldehyde by taking advantage that acetaldehyde has a low boiling point and is soluble in water.
The method in PTL 1, however, has the following problems: the distillation column for separating and removing acetaldehyde is provided with a complicated apparatus; and an expensive material that can correspond to hydroiodic acid produced by hydrolysis of methyl iodide is required to be used to result in the increase in apparatus cost.
In addition, the method of bringing the vapors into contact with water for absorption in PTL 2 has the following problem: methyl iodide and methyl acetate that are useful substances while being present in a small amount are dissolved in water and discharged together with acetaldehyde outside of the system.
It is to be noted that such problems occur with respect to not only acetaldehyde included in an acetic acid-containing liquid but also aldehydes included in various carboxylic acid-containing liquids.
Herein, PTL 3 shows a recovery method by an adsorption method. PTL 3 relates to a method for purifying nitrile, and discloses a method for bringing nitrile into contact with a cation exchange resin, on which a polyvalent amine is supported, to remove aldehyde in nitrile. This method, however, allows other carbonyl compounds that react with an amino group (for example, carboxylic acids and ketones) to be also simultaneously removed together with aldehyde, as described in paragraph [0011] in PTL 3, and therefore a problem of the method is that when used for removing aldehyde in a carboxylic acid-containing liquid, it causes the reduction in yield of carboxylic acid to thereby achieve no desired effects.